The use of active-array-antenna systems (AAS) is becoming increasingly popular in communication systems, for example where radio frequency (RF) components such as power amplifiers and transceivers are integrated with an array of antenna elements. FIG. 1 shows an example of such a system, comprising an array of antenna elements 101 to 10N, power amplifiers 201 to 20N, (for example each comprising a transmit power amplifier, e.g. 201A, and a receive power amplifier, e.g. 201B), and baseband processing circuitry 30 for processing signals being transmitted to, or received from the antenna array. Such a system can be controlled to provide beam forming, for example a beam 40.
Active-array-antenna systems offer several benefits compared to traditional deployments that utilize passive antennas connected to transceivers through feeder cables, for example such as those shown in FIG. 2. In the passive antenna array system depicted in FIG. 2, baseband signals from/to the baseband processing circuitry 30 are boosted using power amplifiers 501/502, which are connected to the antenna elements 101 to 10N via a passive power combiner/driver and phase shifting circuit 60 using long feeder cables. It is noted that, for simplicity, other blocks such as digital analog converters (DACs), Oscillators, up/down converters etc., have been omitted from FIG. 2.
By using active antenna arrays of the type shown in FIG. 1, not only are cable losses reduced, leading to improved performance and reduced energy consumption, but the installation is also simplified, and the required equipment space is therefore reduced.
There are many applications of active antennas, for example cell specific beamforming, user specific beamforming, vertical sectorization, massive multiple-input-multiple-output (MIMO), elevation beamforming etc., and these may also be enablers for further-advanced antenna concepts, such as deploying a large number of MIMO antenna elements at a network node, such as an eNode B. Consideration is also being given to study full-dimensional MIMO (FD-MIMO), which explores the feasibility of increasing the number of transmit antennas to 16/32/64 for various purposes.
Power amplifiers are typically used in active antenna systems. In general, power amplifiers need to be operated in the non-linear region for achieving good efficiency. FIG. 3 shows a typical AM/AM curve for a power amplifier. We can observe that the input/output curve is highly non-linear.
However, when the power amplifier operates in the non-linear region, some of the signals are leaked to the other frequency bands. FIG. 4 shows an example of the spectral regrowth that can occur due to the non-linearity of a power amplifier (curve 41 corresponding to that of an ideal power amplifier, and curve 43 corresponding to that of a realistic power amplifier with non-linearity).
Adjacent Channel Leakage Ratio (ACLR) can be used as a metric to measure the leakage due to non-linear power amplifiers. In FIG. 4 the ACLR for an ideal power amplifier is around −78.1 dBc, while for a realistic power amplifier (with non-linearity), the ACLR is around −41.1 dBc.
One method to compensate for the non-linearity of the power amplifier is to distort the input signal to the power amplifier, such that the output signal from the power amplifier is transformed to be close to what it would have been if the power amplifier would have been linear. Digital pre-distortion techniques (DPD), for example, operate in this way.
FIG. 5 shows the spectral regrowth when DPD is used, whereby curve 41 corresponds to that of an ideal power amplifier, curve 43 corresponds to that of a realistic power amplifier with non-linearity, and curve 45 to that of a power amplifier using DPD. It can be seen that the spectral regrowth is reduced when DPD is applied. In this case the ACLR is improved to be around −60 dBc.
In a MIMO system using AAS, in addition to problems related to non-linear power amplifiers, the signals from the adjacent elements can leak and corrupt the desired signal. Since an AAS base station can be integrated in a limited volume, it is reasonable to assume that mutual coupling between antenna elements is stronger than 30 dB, even in a system not intended to perform cell-specific beam-forming. This is referred to as mutual coupling, crosstalk or antenna port-to-port isolation. In effect, the leaked signal appears as a reverse intermodulation signal at the transceiver output. In particular, if the signals transmitted from the transceivers are uncorrelated (as is likely to be the case to some degree in a MIMO system), then the reverse intermodulation signal can cause disturbances to the operation of control algorithms such as digital predistortion (DPD). If, however, the transceivers are transmitting fully correlated signals, then the impact to DPD is not as severe. It can be noted that if the transmission rank (i.e. number of independently modulated data streams) is the same as the number of transmit branches, the signals are in general uncorrelated, while if a low rank signal is transmitted over many antenna branches, the signals are in general more correlated.
For MIMO systems, when the transmission rank is equal to 1, the signals from the transceivers are perfectly correlated. If the rank is greater than 1, the signals are to some extent uncorrelated. Also, if transmit diversity is applied, then the signals are uncorrelated. The impact due to cross talk can be more severe when the distance between the antenna elements is very small.
FIG. 6 shows an example of the spectral regrowth due to mutual coupling with different mutual coupling values in dB (also shown is the distance between elements). Curve 61 corresponds to no mutual coupling, curve 62 corresponds to a mutual coupling of −34 dB or distance d=1.0λ, curve 63 corresponds to a mutual coupling of −44 dB or distance d=1.5λ, curve 64 corresponds to a mutual coupling of −27 dB or distance d=0.75λ, curve 65 corresponds to a mutual coupling of −11 dB or distance d=0.35λ, and curve 66 corresponds to a non-linear amplifier without DPD. It can be seen that as the mutual coupling value increases (i.e. the distance between the elements decreases), the spectrum moves away from the ideal power amplifier, due to the operation of the DPD being disturbed. This implies that it does not meet the requirements as set by 3GPP.
It should be noted that transceivers driving different radiating elements may experience different levels of coupling. Thus, spectral regrowth effects may be more severe for some transceivers than for others.
To mitigate the impact due to mutual coupling, crossover digital pre distortion (CO-DPD) techniques have been proposed, which take the mutual coupling into consideration in the DPD formulation. Hence the DPD techniques can be divided into two main types.
Type A DPD technique: All conventional DPD techniques fall under this category. These techniques do not take mutual coupling effects into consideration when formulating the DPD coefficients.
Type B DPD technique: This type of DPD technique takes into consideration the mutual coupling and associated reverse intermodulation signals when formulating the DPD coefficients. Another category where the mutual coupling is estimated and cancelled before passing to the DPD also comes under this category.
In general, both the DPD techniques (i.e. those which consider mutual coupling between the antenna elements, as well as those which do not take mutual coupling between antenna elements into consideration) require large computational resources and power at the transmission side. The DPD loop needs to be operated continuously. Thus, in a large MIMO system or in a system with many transmit antennas (antenna elements), running a DPD loop for all the antenna elements can be cumbersome and drain the power.
One solution is to reduce the power of the power amplifier, i.e. use power amplifier back-off. FIG. 7 shows the ACLR value in dBc vs power amplifier power offset with a mutual coupling of 11 dB. It can be observed that to meet the requirement of say −50 dBc the power amplifier back-off value should be at least 3 dB.
However, reducing the power of the power amplifier has the disadvantage of reducing the coverage of a cell. FIG. 8 shows system throughput as a function of power back-off. The simulations are carried out in a 57 sector system simulator. The power amplifier back-off is applied only at the center cell, while maintaining constant power for the remaining 56 cells. The statistics are shown for the center cell, with curve 81 relating to the cell edge, curve 83 relating to the cell average, and curve 85 relating to the cell peak. It can be seen that coverage (usually measured by cell edge user throughput) is almost zero when there is a power amplifier back-off by 3 dB.
FIG. 9 shows the throughput loss in percentage terms, as a function of back-off value (again with curve 81 relating to the cell edge, curve 83 relating to the cell average, and curve 85 relating to the cell peak). It can be seen from FIG. 9 that the loss is more severe at the cell edge than the cell center.